Process for feeding pulp into a blow tank or storage tank

ABSTRACT

A method and apparatus for treating pulp being fed into a blow tank or a storage tank. A preferred embodiment is that the pulp is discharged from a batch digester ( 10 ) and fed either to the upper or lower part of a blow tank ( 20 ), depending on the consistency of the pulp being discharged from the digester.

The present invention relates to a method and apparatus for treatingpulp. A problem resulting in the development of the method and apparatusaccording to the invention becomes especially obvious during the feedand discharge of blow tanks used in connection with batch digesters.Thus, putting it more precisely, the method and apparatus according tothe invention relate to the filling and discharging of various pulptanks and storage containers.

It is known from prior art that a so-called blow tank is used inconnection with batch cooking departments. Contrary to continuouscooking, batch cooking is understood to be a pulp production method inwhich the cooking department comprises several digesters, most commonly5–10 in one cooking department. Each of said digesters in turn is filledwith chips and cooking chemicals which are allowed to effect the chipsfor a certain period of time, after which the digester is discharged toa so-called blow tank. The timing of the feedings and discharges of thedigesters is effected so that the blow tank is filled at fairly regularintervals. The blow tank is meant to function as an intermediatestorage, blow container, of the cooked pulp, from which the pulp isdischarged in an even flow to the next, invariably continuouslyoperating process.

A problem discovered in operating said blow tanks is that theconsistency of pulp being discharged from the blow tank does not remainwithin ranges required by the following process stage, i.e. the brownstock washing department. A reason for this is that each digester hasits own way of discharging depending on e.g. the operator, theuniformity and execution of the cook in general as well as the raw woodmaterial and pulp quality. During the discharge process, the consistencyof the pulp may vary even between zero and ten percent. A reason forthis consistency variation is that in the beginning of the dischargeprocess practically only liquor is discharged from the digester, becauseafter the so-called final displacement which in modern batch cookingprocesses is the last process prior to the discharge of the digester,there is a certain amount of displacement liquor in the bottom part ofthe digester. This is naturally discharged from the digester first,before the actual pulp. During the discharge, and more intensivelytowards the end of the discharge, the pulp has to be diluted in order tomake it flow from the digester to the discharge pump. Just at the finalstage of the discharge, the pulp has to be diluted even more, as thehydrostatic pressure under which the pulp is discharged from the tank issubstantially lower than in the beginning of the discharge due to lowheight of the pulp column.

In the blow tank, the consistency variation often results in channelingof the pulp so that at various locations of the blow tank theconsistency of the pulp alters to a large extent. Although the blow tankis provided with one or several mixers, our experience has shown thatthese are not capable of equalizing the consistency of the pulp in allconditions, but the pulp is discharged from the tank to the followingprocess stage at an unsuitable consistency. In the worst case, theconsistency of the pulp is decreased in the mixing zone of the tank farbelow the desired values.

A second problem consists in filtration of the pulp to form a cake onthe surface of the pulp existing in the blow tank. A direct reason forthis is the lack of motion in the upper part of the mixing zone and thefact that the pulp being discharged from the digester always containsair and gases originating from the cook which are not easily removedfrom the pulp. This phenomenon is also dependent on the quality and typeof pulp. Especially when running the tank in order to empty it orsometimes even in normal running situations, depending on the surfacelevel, said filtrated pulp causes remarkable consistency variations whenentering the mixing zone at the bottom part of the tank, whichconsistency variations can not be equalized in the mixing zone, but thepulp is discharged further at too high a consistency.

FIG. 1 illustrates the consistency variation of pulp during the blow ofone digester. As already stated earlier, the consistency of the pulpvaries to a great extent during the discharge, being low both in thebeginning and in the end due to reasons stated before. In the middlestage of the blow the consistency of the pulp may be in the order of tenpercent. The time interval between two blows is in the order of 20–40minutes depending on the size and number of digesters. In our tests wehave noticed that said interval of about half an hour is sufficient tocause the pulp on the surface in the blow tank to filtrate, i.e.thicken, whereby onto the surface of the pulp existing in the tank arelatively solid, and continuously solidifying, cake is formed.

FIG. 2 a illustrates a case in which the digester is discharged to ablow tank via a conduit arranged at the upper end of the tank. In thiscase it is obvious that the pulp is discharged, if no auxiliary meansdistributing the flow are used, with considerably great force deep intothe pulp already existing in the tank. Thus, in the one hand, the pulpfiltered on the surface in the blow tank cannot at any stage enter themixing zone, except if the tank is being run to empty it or to lower thesurface level, but the pulp starts to be collected on the surface in thetank. At the worst case the pulp may start to deteriorate, if thehardened pulp cake remains in the tank for a long period of time. On theother hand, the dilute pulp discharged deep into the pulp existing inthe tank forms a local agglomeration, which at some stage when the blowtank is being evenly discharged is suddenly discharged into the mixingzone decreasing the consistency of pulp being discharged from the tankto below the desired values.

FI patents 98836 and 100011 deal partly with the same problem andsuggest as a solution various apparatuses by means of which it ispossible to somewhat restrict the downward flow of pulp entering themixing zone or on the other hand fill the blow tank better than before.

Said better filling of the blow tank is illustrated in FIG. 2 b where itcan be seen how the apparatus according to FI patent 100011 is used tofeed pulp into the tank via the bottom part of the tank to a certainaltitude. Although said solution does help in many problems related tothe filling of the tank, FIG. 2 b illustrates also a situation wheredilute pulp is being discharged to the tank through the apparatus. Thischannels directly from the inlet opening of the feeding apparatustowards the mixing zone, resulting in principle in the same kind ofsituation as the solution according to FIG. 2 a.

Naturally, when treating pulp, problems of the same kind appear not onlyin connection with blow tanks but elsewhere as well, e.g. in connectionwith pulp storage tanks at various stages of the pulp treatment process.

Said problems are suggested to be solved by an arrangement where part ofthe pulp is discharged into the tank via its upper part and part of thepulp through the lower part of the tank. Further, the pulp dischargedvia the upper part of the tank is in the upper part of the tankdistributed evenly onto the whole cross section of the tank, wherebyeven the pulp discharged from the upper part of the tank is not capableof penetrating deep into the pulp existing in the tank, but stays on thesurface of the pulp, thus ensuring that the consistency of the pulpremains uniform at various locations in the storage tank and that thepulp flows evenly at a relatively uniform consistency downwards to themixing zone.

The operating model described above solves a third problem, too, viz. adisadvantage related to the energy consumption of the filling of blowand storage tanks. It has namely been noticed that especially when thetanks are filled through an inlet opening arranged at the upper end ofthe tank only, remarkable amount of pumping energy is lost due to greatfluctuations in the pulp level in the tank. Pumping the pulp to thealtitude of the upper end of the tank and letting it drop from there tothe bottom of the tank is mere waste of energy. By utilizing thesolution according to the invention described above, part of the tank,according to a preferred example half of the tank, i.e. the bottom partof it, is filled substantially through a feed inlet at the bottom of thetank and only the upper part of the tank through a feed inlet locatedessentially at the upper end of the tank. When filling the tank throughthe bottom thereof, at the bottom of the tank there is preferablyprovided a filling device according to FI-patent 100011 and accordinglyat the upper end of the tank there is preferably provided a fillingdevice according to FI-application 971330. With the described method,about a third of the pumping energy is saved compared to filling thetank through the upper end, naturally depending on the average altitudelevel of the pulp in the tank.

Other characterizing features of the method and apparatus according tothe invention are disclosed in the appended claims.

In the following, the method and apparatus according to the inventionare explained in more detail with reference to the appended figures, ofwhich

FIG. 1 illustrates the change of discharge consistency in a dischargeprocess of a prior art batch digester as a function of time,

FIG. 2 a illustrates a blow arrangement of a prior art batch digester,

FIG. 2 b illustrates a blow arrangement of a second prior art batchdigester,

FIG. 3 illustrates a solution according to a preferred embodiment of theinvention applied to the blow arrangement for a batch digester.

FIG. 1 illustrates the consistency variation of the pulp during the blowof one digester. As already stated, the consistency is low both in thebeginning and in the end of the discharge. In about the middle stage ofthe blow the consistency of the pulp may be in the order of about tenpercent. The time interval between two blows is about 20–40 minutes,depending on the size and number of digesters. In our studies we havenoticed that said interval in the order of about half an hour issufficient to cause the pulp on the surface in the blow tank tofiltrate, or thicken, whereby a relatively solid, and continuouslysolidifying, pulp cake is formed onto the surface of the pulp alreadyexisting in the tank. One has to notice about the described figure thatit is only an example of a batch digester and a blow as run by oneoperator. That is, each mill and each operator there and even eachdigester may produce different consistency profiles as a function ofdischarge time.

FIG. 2 a illustrates a case in which the digester 10 is discharged intoa blow tank 20 through a conduit 22 arranged at the upper end, moregenerally speaking in the upper part, of the tank. In this case it isobvious that if no auxiliary means distributing the flow are used, thepulp is discharged with a considerable force deep into the pulp alreadyexisting in the tank 20. And so, on the one hand, the pulp S filtratedon the surface in the blow tank 20 can not in any stage, except whenrunning the tank 20 to empty it or to just lower the surface level,enter the mixing zone 24 located in the lower part of the tank, butstarts to be collected on top of the pulp existing in the tank 20 and toharden, forming a solid cake. The dilute pulp, sometimes consisting ofdilution liquor only, discharged deep into the pulp in the tank 20, onthe other hand, forms a local agglomeration which in some stage of theeven discharge of the blow tank is suddenly discharged into the mixingzone 24, causing the consistency of the pulp being discharged from thetank 20 to line 26 to decrease to a value below the desired values.

FI patents 98836 and 100011 deal partly with the same problem andsuggest as a solution various apparatuses by means of which it ispossible to somewhat restrict the downward flow of pulp entering themixing zone 24 or, on the other hand, fill the blow tank 20 better thanbefore. Said better way of filling the blow tank 20 is illustrated inFIG. 2 b showing how by means of an apparatus 28 according to FI-patent100011 pulp is fed into the tank 20 to a certain altitude via the bottompart of the tank. Said patent even suggests various possibilities offeeding pulp to different altitudes of the tank. Although said solutiondoes help in many problems related to the filling of the tank, FIG. 2 balso illustrates a situation where dilute pulp is discharged into thetank 20 through the apparatus 28. This channels directly from the inletopening of the feeding apparatus 28, which inlet opening is locatedabove the mixing zone 24, towards said mixing zone, resulting inprinciple in the same kind of situation as the solution according toFIG. 2 a. In other words, if the feeding apparatus 28 is not capable offeeding all pulp entering the tank reliably on top of the pulp alreadyexisting in the tank, or at least into the surface layer of it, apermanent pulp layer S is left in the tank 20, which layer, constantlyfiltrating, gets harder and thicker. As to the feeding apparatus 28, itmay be further stated that, even if it were capable of discharging thepulp into the surface layer of the pulp in the tank, it can not preventthe pulp on the perimeter of the tank from filtrating, whereby it isquite possible that the surface pulp layer S on the perimeter of thetank remains in the tank for a long time, while the pulp in the centralpart of the tank is channeled relatively directly to the dilution zone.

FIG. 3 illustrates how each batch digester 10 in the cooking departmentis connected via a flow path 32 to a blow pump 34, which further feedsthe pulp to be discharged to a pressure tube 36. A valve 38 is arrangedin the pressure tube 36 by means of which valve the pulp flow may bedistributed either to a feed pipe 40 or 42 of the blow tank. The feedpipe 40 leads to the upper part of the blow tank 20, preferably, asillustrated in the figure, to the upper end, and feed pipe 42 to thelower part of the blow tank 20. Of course, in connection with the upperpart of the tank, the pulp may be introduced via a pipe extendingthrough a side wall of the tank as well, but even in that case so thatthe actual pulp feed from the pipe into the tank is practicedsubstantially in the vicinity of the upper end of the tank. The feedingapparatus 28 in the lower part of the tank 20 is preferably an apparatusaccording to FI patent 100011 illustrated already in connection withFIG. 2 b, by means of which apparatus the feeding of the pulp into thetank is effected vertically upwards. In accordance with the invention,the valve 38 is controlled as a function of the consistency of the pulppreferably so that through the lower part of the blow tank 20, thickpulp is fed into the tank 20. The consistency range may be determined tobe e.g. over 1–3%. Respectively, dilute pulp, having a consistency ofe.g. below 1–3%, is fed into the blow tank 20 through the upper partthereof.

According to a preferred embodiment of the invention, the feedingapparatuses of the upper part of the tank 20 comprise an apparatusschematically illustrated under reference number 44, which apparatusdistributes the pulp essentially to the whole cross section of the tank20. This apparatus may be for example of the kind described in FI patentapplication 971330. The main objective of the apparatus 44 is to directthe flow of the pulp fed into the tank so that the flow can not bedischarged with a great force directly downwards penetrating deep intothe pulp already existing in the tank 20, but is distributed on top ofthe pulp already existing in the tank 20, thus keeping the consistencyof the surface layer L of the pulp low. By leading the dilute pulp inthis way onto the pulp in the tank 20, essentially onto the whole crosssection of the tank 20, it can be considered sure that, on the one hand,the pulp L on the pulp column can at no location of the cross section ofthe tank 20 be filtered excessively and, on the other hand, no part ofthe pulp is left in the tank for too long a time, but is evenlydischarged from the tank 20.

One way of controlling said valve 38 is to utilize the consistencyprofile of FIG. 1. Thus, e.g., if it is desirable to feed the pulp at aconsistency of less than 2% through the upper part of the tank 20 ontothe pulp already existing in the tank 20 and the pulp at a consistencyof over 2% through the bottom part of the tank 20, the valve 38 iscontrolled so that when the discharge of the digester 10 initiates, thevalve will immediately start to direct the pulp flow to the feed pipe 40going to the upper part of the tank 20. According to FIG. 1, as about 5minutes have passed from the beginning of the discharge of the digester10, the valve 38 is turned either manually or automatically to aposition by which the blow flow is guided via the feed pipe 42 throughthe lower part of the tank 20, preferably utilizing the feedingapparatus 28, into the tank 20. Further, when about 25 minutes havepassed from the beginning of the discharge of the digester 10, the valve38 is turned back to a position by which the blow flow is guided to theupper part of the tank 20.

Another possible method of controlling the valve 38 is to arrange aconsistency detector in the blow line 32, 34 or 36 of the digesters 10,which detector guides the valve 38 automatically or on the basis ofwhich the valve 38 is guided manually. In that case it is obvious thatmonitoring the consistency relatively easily leads to automaticalcontrol of the valve 38. And further, it is possible to monitor theconsistency by monitoring e.g. the power requirement of the blow pump34. It is a known fact that the power requirement of a centrifugal pumpis directly proportional to the consistency of the pulp. To put itdifferently, when the input power of the pump 34 rises over a certainvalue, it means that the consistency in line 32–36 has increased overthe desired value and the valve must be turned to another position. Theratio of the input power of the pump to the consistency of the pulp maybe determined e.g. in connection with the implementation ormanufacturing of the pump or even with test runs performed whendesigning the pump model.

It is also obvious that the valve 38 may also be replaced by valvespositioned in both feed pipes 40 and 42, which valves are controlled toeffect the model of functioning described above. Further, if there is adoubt that the valves may get clogged, it is possible to arrange at thebeginning end of feed pipes 40 and 42, as a matter of fact in thedistribution point of the flow a special distribution device, e.g. theone described in U.S. Pat. No. 4,964,950 specifically planned to be usedin pipelines for thick pulp.

There are still other possibilities for determining the consistency. Onemethod is to use in the discharge pipe a consistency transducer that inprinciple belongs to prior art, but the object of application thereofhas proved to be very complicated due to e.g. the gases existing in thepulp. One possible option is the use of newer consistency meters basedon e.g. ultrasound or radioactive signals, which provide forsufficiently reliable consistency detection. It is worth rememberingthat the consistency value in this operation needs not be anything elsethan reference value.

Still another method is to arrange in connection with the discharge pumpinformation technology to such an extent that on the basis of theoperating parameters, e.g. power input, of the pump it is possible todetermine the changes in the consistency of the pulp. However, air andother gases in large amounts confuse the situation.

The amount of gas in the pulp pumped by the pump may be estimated if thepump is provided with gas-separation means. In that case, by e.g.keeping the suction of the gas-removal constant, on the basis of earliertest runs the total amount of gas in the pulp may be calculated from theamount of gas being discharged due to said suction.

Yet another possibility is a pre-programmed digester discharge curve andthe determining of the consistency by calculating on the base of theliquor amount passing through the dilution liquor valves, which methodsare in principle possible, but somewhat less exact than the techniquesdescribed above.

Still further ways of determining the consistency are offered by variousdevices based on process tomography which allow for the determination ofboth the consistency and the gas-content of the pulp.

In addition to the blow of a batch digester, the consistency of the pulpmay vary during the process e.g. in connection with various presses,washers and/or various dilution devices to such an extent that the useof the apparatus according to the invention for filling the tank iswell-founded.

If it comes to the solution of energy consumption only, the valve 38 orvalves substituting it may be controlled based on e.g. informationreceived from a surface level indicator of the tank.

As noticed from the above, a method and apparatus have been developedwhich solve the problems of prior art and by means of which e.g. thedischarge of a batch digester and the further treatment of pulpdischarged from the digester to the blow tank may be optimized so thatthe consistency of pulp going from the blow tank to brown stock washingremains within the value required by the brown stock washer. One has tonotice, though, that our invention is not limited to the discharge of abatch digester to a blow tank, nor to the use of any special apparatusmeant for consistency measurement. It has to be noticed that thedescribed application of the invention to batch digesters must beunderstood as an exemplary application with no intention to restrict theinvention to a narrower scope than described in the claims. Thus, theinvention is applicable to be used in connection with any blow, storageor the like tank in the process, in which the same type of problems havebeen found. And accordingly, some preferable embodiments of theinvention only utilize a process variable that is effected by theconsistency of the pulp. In other words, numeric data on the consistencyitself is not needed at any stage when applying the invention, but onlyan indication of the change of the consistency is needed. Thus, it isenough to arrange in the pulp flow pipe or in connection with it adevice or member responding to the consistency of the pulp flowing inthe pipe.

1. A method of treating pulp, by which method pulp is discharged from aprocess apparatus and fed into a blow tank or storage tank, wherein themethod comprises: discharging pulp from the process apparatus andfeeding the pulp to the blow tank or storage tank, wherein the pulpbeing fed is of varying consistency; feeding the pulp either to an upperpart of the tank or to a lower part of the tank depending on aconsistency of the pulp being fed from said process apparatus in such amanner that when the pulp is at a consistency below a certainpredetermined discharge consistency it is fed into said tank through theupper part of the tank and when the pulp is at a consistency above saidpredetermined discharge consistency the pulp is fed into said tankthrough the lower part of the tank.
 2. A method according to claim 1,further comprising controlling the feeding of pulp to the upper part andlower part of the tank by a consistency detector arranged in a dischargetube of said process apparatus.
 3. A method according to claim 2,wherein said consistency detector is a blow pump.
 4. A method accordingto claim 1, wherein said process apparatus is a batch digester.
 5. Amethod according to claim 1, wherein the feeding of pulp is controlledaccording to a pre-determined consistency profile.
 6. A method accordingto claim 5, wherein said predetermined consistency profile varies as afunction of time, and whereby said feeding of pulp is controlled basedon time passed from a beginning of an operation of the digesterdischarge.
 7. A method according to claim 1, wherein the feeding of pulpthrough the upper part of the tank is distributed onto a whole crosssection of the tank.
 8. A method according to claim 1, wherein thefeeding of pulp through the upper part of the tank is distributed on topof pulp in the tank.
 9. A method of treating pulp, by which method pulpis discharged from a process apparatus and fed into a blow tank orstorage tank, wherein the method comprises: discharging pulp from theprocess apparatus and feeding the pulp to the blow tank or storage tank,wherein a consistency of the pulp changes in time; feeding the pulpeither to an upper part of the tank or to a lower part of the tankdepending on the consistency of the pulp being fed from said processapparatus in such a manner that when the pulp is at a consistency belowa certain predetermined discharge consistency it is fed into said tankthrough the upper part of the tank and when the pulp is at a consistencyabove said predetermined discharge consistency the pulp is fed into saidtank through the lower part of the tank.
 10. A method according to claim9, further comprising controlling the feeding of pulp to the upper partand lower part of the tank by a consistency detector arranged in adischarge tube of said process apparatus.
 11. A method according toclaim 10, wherein said consistency detector is a blow pump.
 12. A methodaccording to claim 9, wherein said process apparatus is a batchdigester.
 13. A method according to claim 9, wherein the feeding of pulpis controlled according to a pre-determined consistency profile.
 14. Amethod according to claim 13, wherein said predetermined consistencyprofile varies as a function of time, and whereby said feeding of pulpis controlled based on time passed from a beginning of an operation ofthe digester discharge.
 15. A method according to claim 9, wherein thefeeding of pulp through the upper part of the tank is distributed onto awhole cross section of the tank.
 16. A method according to claim 9,wherein the feeding of pulp through the upper part of the tank isdistributed on top of pulp in the tank.
 17. A method of treating pulp,by which method pulp is discharged from a process apparatus and fed to ablow tank or storage tank, wherein the method comprises: dischargingpulp from the process apparatus and feeding the pulp to the blow tank orstorage tank, wherein the pulp being fed at various times during themethod has a dilute consistency and a thick consistency; feeding thepulp either to an upper part of the tank or to a lower part of the tankdepending on a consistency of the pulp being fed from said processapparatus such that the pulp at the dilute consistency it is fed to theupper part of the tank and pulp at the thick consistency is fed to thelower part of the tank.
 18. A method according to claim 17 wherein thedilute consistency is a pulp consistency below a predeterminedconsistency and said thick consistency is above the predeterminedconsistency.
 19. A method according to claim 17, further comprisingcontrolling the feeding of pulp to the upper part and the lower part ofthe tank by a consistency detector arranged in a discharge tube of saidprocess apparatus.
 20. A method according to claim 17, wherein saidconsistency detector is a blow pump.
 21. A method according to claim 17,wherein said process apparatus is a batch digester.
 22. A methodaccording to claim 17, wherein the feeding of pulp to either the upperpart or the lower part of the tank is controlled according to apre-determined consistency profile of the pulp.
 23. A method accordingto claim 22, wherein said predetermined consistency profile varies as afunction of time, and whereby said feeding of pulp is controlled basedon time passed from a beginning of an operation of a digester discharge.24. A method according to claim 17, wherein the feeding of pulp throughthe upper part of the tank is distributed over a cross section of thetank.
 25. A method according to claim 17, wherein the feeding of pulpthrough the upper part of the tank is distributed over a top of the pulpin the tank.